The present invention relates to electrocatalyst compositions. More specifically, the invention relates to electrocatalysts which can be used on the anode of a fuel cell to oxidize alcohol, in particular, ethanol.
A fuel cell is an electrochemical device with an anode and a cathode that converts chemical energy provided by a fuel and an oxidant into electricity. An electrolyte is required which is in contact with both electrodes and which may be alkaline, acidic, solid or liquid. The basic process is highly efficient and essentially pollution-free. Also, since single fuel cells can be assembled in stacks of varying sizes, systems can be designed to produce a wide range of output levels.
As an energy conversion device, the fuel cell is distinguished from a conventional battery by its fuel storage capacity. Unlike a battery which consumes internally stored fuel and needs to be either discarded or recharged after a certain time, fuel is fed to the fuel cell from an external source giving the fuel cell a practically unlimited storage capacity. Also, the fuel cell is distinguished from a battery in that its electrodes are catalytically active.
Current is generated by reaction on the fuel cell electrode surfaces, which are in contact with an electrolyte. The fuel is oxidized at the anode and gives up electrons to an external electrical load. The oxidant accepts electrons and is reduced at the cathode. Ionic current through an electrolyte completes the circuit.
In many fuel cell systems, a hydrogen fuel is produced by converting a hydrocarbon-based fuel such as methane, or an oxygenated hydrocarbon fuel such as methanol, to hydrogen in a process called reforming. This reformate fuel contains, in addition to hydrogen, high levels of carbon dioxide, usually around 25%. The reformate fuel also contains small amounts of impurities, such as carbon monoxide, typically at levels of around 1%.
Other fuel cells, called “direct” or “non-reformed” fuel cells oxidize fuel high in hydrogen content directly, without the hydrogen first being separated by a reforming process. It has been known since the 1950s that lower primary alcohols, particularly the C1-C5 alcohols, can be oxidized directly (i.e., without reformation to H2+CO or H2+CO2) at the anode of a fuel cell. Methanol and ethanol are particularly useful.
Because compactness is critical to the commercial viability of utilizing a fuel cell as an energy source in such items as electric automobiles, the ability to oxidize alcohol as a fuel directly without having to also utilize a reformer is important. Serious drawbacks have also been encountered in the storage of hydrogen for use in fuel cells. Thus, a “direct” fuel cell, such as an ethanol fuel cell, is advantageous in that it is compact and no energy is used up in reformation. Further, the fuel is easily stored in liquid form, is high in hydrogen content, is highly reactive in a fuel cell, and is economically viable.
In a typical ethanol fuel cell, ethanol is oxidized to produce electricity, heat, water, and carbon dioxide. The goal in ethanol fuel processing is complete ethanol oxidation for maximum energy generation as shown in the equation:Anode: CH3CH2OH+3H2O→2CO2+12H++12e−Cathode: 3O2+12e−+12H+→H2ONet: CH3CH2OH+3O2→2CO2+3H2O.  (1)
In the absence of an electrocatalyst, a typical electrode reaction occurs, if at all, only at very high overpotentials. Thus, the oxidation and reduction reactions require catalysts in order to proceed at useful rates. Catalysts that promote electrochemical reactions, such as oxygen reduction and hydrogen oxidation in a fuel cell, are often referred to as electrocatalysts. Electrocatalysts are important because the energy efficiency of any cell is determined, in part, by the overpotentials necessary at the cell's anode and cathode.
Platinum (Pt), an expensive metal, is the best catalyst for many electrochemical reactions, including ethanol oxidation. A major obstacle in the development of ethanol fuel cells is the loss of electrochemical activity of even the best electrocatalyst due to “poisoning” by carbon monoxide (CO), and the accumulation of other intermediates such as acetate and acetaldehyde caused by the inability of Pt to break C—C bonds except at very high potentials. CO is an intermediate in the oxidation of ethanol to carbon dioxide (CO2). The CO molecule is strongly adsorbed on the electroactive surface of the electrode, obstructing the oxidation of new fuel molecules. It is well known that CO, even at levels of 1-10 ppm, is a severe poison to platinum electrocatalysts and significantly reduces fuel cell performance.
Various attempts have been made to find a solution to the CO poisoning problem. For example, Reddy et al., U.S. Pat. No. 5,132,193 discloses the use of gold crystals for the oxidation of alcohol. Yepez, U.S. Pat. No. 5,804,325, discloses the use of deliberately occluded hydrogen in the anode to chemisorb the poisons. Various combinations of metals have also been employed as an electrocatalyst material in an attempt to avoid or minimize the CO poisoning problem. For example, Ma et al. U.S. Pat. No. 5,702,836 discloses an electrocatalyst obtained by combining platinum oxides and iron oxides to form Pt/Fe particles in a colloidal solution.
In spite of the foregoing, prior attempts to solve the problem of CO poisoning and breaking the C—C bond at the anode of ethanol fuel cells have been unsuccessful. Prior attempts to avoid the problem have proven to be too expensive, ineffective, or impractical to be commercially viable. Thus, there remains a need for electrocatalysts that can be used on the anode for alcohol oxidation in fuel cells and that are resistant to CO poisoning and can break the C—C bond at low potentials.